For restoration of tooth structures (enamel, dentin, and cementum) damaged by dental caries or the like, restorative filling materials such as filling composite resins and filling compomers, or crown restoration materials such as metal alloys, porcelains, and resin materials, are usually used. In general, however, restorative filling materials and crown restoration materials (both may be collectively referred to as “dental restorative materials” in the present description) themselves have no adhesive properties to tooth structures. Therefore, various adhesion systems employing adhesives are conventionally used for adhesion between tooth structures and dental restorative materials. As conventional adhesion systems widely used, there are known so-called acid-etching adhesion systems in which the surface of a tooth structure is subjected to an etching treatment using an acid etching agent such as an aqueous phosphoric acid solution, followed by applying a bonding agent that is an adhesive and then by adhering the tooth structure and a dental restorative material together.
In addition, so-called self-etching adhesion systems are known as adhesion systems that do not use acid etching agents. Self-etching adhesion systems that had been predominantly used in the past are two-step adhesion systems in which a self-etching primer containing an acidic monomer, a hydrophilic monomer, and water is applied to the surface of a tooth structure, followed by applying a bonding agent containing a crosslinkable monomer and a polymerization initiator without washing with water. In recent years, however, one-step adhesion systems using a one-part dental adhesive having the functions of both a self-etching primer and a bonding agent have been widely used.
In general, such a one-part dental adhesive contains constituents of a self-etching primer and/or a bonding agent; specifically, such a one-part dental adhesive contains: a monomer containing an acidic group such as a carboxylic acid group, a phosphonic acid group, a thiophosphoric acid group, or a phosphoric acid group; a hydrophilic monomer containing a hydroxyl group or the like; a crosslinkable monomer; water; and a polymerization initiator.
Key factors in adhesion to enamel of a tooth structure are: a physical interaction with minute irregularities of the enamel surface decalcified; and a chemical interaction with apatite in the enamel. As for adhesion to dentin of a tooth structure, key factors are physical and chemical interactions with a hydrophilic collagen layer exposed as a result of decalcification. Monomers having a hydrophilic group play an important role in the chemical interaction with apatite in enamel and the chemical interaction with a collagen layer in dentin.
The monomers having a hydrophilic group include acidic group-containing monomers and hydrophilic monomers. It has been a common practice to use, as such monomers, an acidic group-containing (meth)acrylate compound and an water-soluble (meth)acrylate compound containing a hydroxyl group. Meanwhile, there has been proposed a one-part dental adhesive in which an acidic group-containing (meth)acrylate compound and a (meth)acrylamide compound having an amide group are used as the monomers having a hydrophilic group. The use of such a one-part dental adhesive has been reported to improve the storage stability and to provide high adhesive properties to dentin and enamel.
For example, Patent Literature 1 proposes a one-part dental adhesive containing an acidic group-containing polymerizable monomer, a water-soluble (meth)acrylamide compound, water, a curing agent, and a crosslinkable polymerizable monomer. In Patent Literature 1, monofunctional (meth)acrylamide compounds are mentioned as examples of the water-soluble (meth)acrylamide compound. According to the study by the present inventors, however, it has been found that the one-part dental adhesive described in Patent Literature 1 still has room for improvement in terms of adhesive properties to dentin, and also has a problem in that its storage stability may be insufficient depending on the type of the monofunctional (meth)acrylamide compound used.
Patent Literature 2 proposes a one-part dental adhesive containing: a polymerizable acidic phosphoric acid ester monomer having a particular structure; a polymerizable acidic monomer having a particular structure; and a polymerizable N-substituted alkylacrylic or acrylic acid amide monomer. It is disclosed that a monofunctional (meth)acrylamide compound or a polyfunctional (meth)acrylamide compound may be contained in the adhesive. In addition, Patent Literature 3 proposes an adhesive containing: at least one acidic (meth)acrylamide monomer having two or more polymerizable groups; an acidic group-containing monomer; a polymerization initiator; and a monofunctional or polyfunctional non-acidic (meth)acrylamide monomer. According to the study by the present inventors, however, it has been found that the adhesives described in Patent Literature 2 and 3 may have poor adhesive properties to either or both dentin and enamel depending on their compositions. In addition, the adhesives described in Patent Literature 2 and 3 have been found to have a problem in that the stability against ambient light is low and the allowable operation time is short.